This invention relates to processes for the production of derivatives of cellulose more particularly to process for preparing cellulose ethers.
In the preparation of many cellulose derivatives, cellulose pulp is reacted with sodium hydroxide to form alkali cellulose. The alkali cellulose is then further reacted with compounds such as alkyl halide, aralkyl halides, alkylene oxides, sodium chloracetate and others to form the desired cellulose derivatives. Alkali cellulose, however, is very susceptible to degradation and the cellulose molecule tends to cleave during processing. Cleavage of the cellulose molecule leads to a lower molecular weight product. This reduction in molecular weight leads to a product which, when in solution, exhibits a low viscosity. An additional problem is that the amount of degradation of the alkali cellulose varies from production run to production run making it difficult to consistently produce a cellulose ether having the desired solution viscosity.
It has been long believed that said degradation of alkali cellulose is due to the reaction thereof with oxygen present in the reaction vessel. However, previous attempts to control the degradation of alkali cellulose by removing the oxygen from the reaction vessel in which the alkali cellulose is prepared and reacted to form cellulose derivatives have failed to give predictable, consistent product viscosities. For example, one commonly used method is to, prior to reacting cellulose pulp with caustic to form alkali cellulose, evacuate the head space of the reactor and then flood the evacuator head space with a gas containing no oxygen. By repeating this process one or more times, it is expected that the amount of oxygen in the head space can be reproducibly controlled to less than about 10.sup.-5 moles/liter of head space, said amount of oxygen varying by less than about 10 percent from production run to production run. Unfortunately, however, products made using this process exhibit vixcosities ranging from 0.5 to 2 times the desired viscosity.
Since it is believed that oxygen degrades alkali cellulose through a free radical mechanism, it has also been attempted to control said degradation through the use of various free radical inhibitors. However, the use of free radical inhibitors does little to control the solution viscosities of the resulting cellulose derivatives.
Because of the fluctuation in the solution viscosities of cellulose derivatives made by known processes, significant time, equipment and expense are required to blend cellulose ether products from different production runs to obtain a final product which is within product specifications. Accordingly, a process whereby cellulose derivatives can be consistently prepared having a predetermined solution viscosity is highly desirable.